In most printed circuit (wire) assembly boards, the use of a straddle mount connector is preferred owing to the robustness of the assembly, space limitations and other constraints. A double sided straddle mount connector typically includes a housing with one or more rows of connector leads extending from one side of the housing for making electrical contact with the respective contact pads; on both surfaces of a printed circuit board (PCB), whereas a single sided straddle mount connector has all its connector leads in contact with only one surface of the PCB.
Whilst much has been done to improve the methods of mounting semiconductor devices to a PCB, the same cannot be said for straddle mount connectors. Compared to components such as semiconductor integrated-circuit devices which can be placed on the PCB surface by some pick-and-place action, the mounting of a straddle mount connector is complicated by the fact that there will be a certain amount of sliding of the connector leads against the PCB surfaces as the connector is pushed against the edge of the PCB. The conventional method is to apply solder to one PCB surface and then to flip the PCB over for solder to be applied to the other PCB surface. The connector is then forced into position along the edge of the PCB before the assembly is sent for solder reflow.
One example of a connector is described by Pauza in the U.S. Pat. No. 6,056,603 issued May 2, 2000. Pauza suggests that an alternating up and down configuration of the connector leads allows for excellent position control between the connector leads. However, as in other double sided straddle mount connectors, such as that proposed by Martens et al in the U.S. Pat. No. 4,734,042 issued Mar. 29, 1998, at least two solder printing processes are required for both surfaces of the PCB.
One way of getting around the need to carry out solder printing on both surfaces of the PCB is to use the method taught by Socha in the U.S. Pat. No. 5,600,102 issued Feb. 4, 1997. Socha suggests the use of a solder preform which can be wrapped about the edge of a PCB to provide solder on both surfaces of the PCB. Taking into consideration the cost of purchasing the solder preform and of introducing new processes in the existing manufacturing line to wrap the solder preform onto the PCB, this does not seem to be a cost-saving solution for the manufacturer.
Significant savings can of course be achieved if the number of printing processes can be reduced. One way of doing so is disclosed by Ueltzen in the U.S. Pat. No. 5,261,989 issued on Nov. 16, 1993 and entitled "Straddle Mounting an Electrical Conductor to a Printed Circuit Board". In this patent, additional solder is deposited in reservoirs on one PCB surface and then ducted to the second PCB surface. However, in reality, it is found that the solder does not travel well across the second PCB surface to the locations where bond formation is desired. In particular, there is inadequate fillet formation around the connector leads further away from the reservoirs, leading to poor electrical contact.
Furthermore, the area that is taken up by the reservoirs is considered underutilized as it cannot be used for mounting other devices. As the demand for miniaturization and more integrated products increase, especially in the area of consumer electronics, there is a need to conserve the valuable real estate on the PCB. If all the required devices cannot be mounted to one surface of the PCB and some have to be mounted to the second surface, this means that two solder printing processes have to be carried out. The result is that the original purpose of having the reservoirs is not achieved, and ironically, manufacturing cost is increased.
An alternative is to use single sided straddle mount connectors such as one described by Baxter et al. in the U.S. Pat. No. 5,897,386 issued Apr. 27, 1999 entitled "Single-Sided Electronic Connector and Method of Assembly". Since all the leads of the connector contact only one surface of the PCB, a second process to deposit solder on the second surface of the PCB becomes unnecessary. Unfortunately, the required pitch of the leads in a single sided connector tends to create serious solder defects such as bridging and insufficient solder. Bridging occurs when solder is pushed aside by the connector leads as the connector is assembled to an edge of the PCB. The solder for neighboring leads may thus merge and create a short in the circuit. In addition, the solder from a first contact pad may be drawn to the solder at an neighboring contact pad because of surface tension effects. The first contact pad is thus left with insufficient solder to form a good joint between the contact pad and the connector lead. In addition, single sided connectors are generally not favored over double sided connectors because they tend to be associated with PCB warpage problems.
It is generally difficult to reduce the number of solder printing processes without sacrificing product quality or increasing manufacturing costs. There is a clear need for an improved straddle mount assembly that facilitates more efficient use of the limited surface area on the PCB at reduced manufacturing costs, whilst producing more consistent quality products. These and other advantages are offered by the present invention, which at the same time provides a solution to other problems faced by the prior art.